1. Field of the Invention
The present invention relates to a defect inspection device for metal rings. More particularly, the present invention relates to a defect inspection device suitable for inspecting the presence of end face defects on metal rings which are parts that constitute a V-belt type of a Continuously Variable Transmission belt (hereinafter denoted as “CVT belt”) mounted in a vehicle, such as an automobile.
2. Description of the Related Art
As a CVT belt, for example, there is a known CVT belt structure which laminates a plurality of thin metal rings in a stack of approximately 0.2 mm in thickness to which steel elements are consecutively attached.
FIG. 10 is an outline view of a CVT belt. In this diagram, a CVT belt 1 is constructed by assembling two laminated bands of a layered belt 3 that contain a stack of a number of metal rings 2 (for example, a laminated band composed of about 12 endless layers) which are supported by a layered element 5 composed of a large number of steel elements 4 (for example, about 400 elements).
FIG. 11 is an outline diagram of the manufacturing process of the CVT belt 1. Referring to this drawing, the end parts 6a of an “ultrahigh strength steel” thin sheet 6 (described later in detail) are welded together and a drum 7 is formed. Next, the drum 7 is cut into round slices of predetermined width and rolled to create metal rings 2 of a basic peripheral length.
Next, after performing a solution treatment, etc. to each of the above-mentioned metal rings 2, a peripheral length correction process is performed that provides the necessary peripheral length corresponding to the lamination location on the CVT belt 1. Here, “peripheral length” means the circumference length of the metal rings 2. The peripheral lengths of the metal rings 2 are subtly different for each lamination stacked position in the CVT belt 1. For example, the outermost periphery side is slightly longer and the innermost periphery side is slightly shorter.
After the peripheral length correction process and further performing aging treatment, nitride treatment, etc. to increase the hardness of the metal rings 2, the presence of surface defects in the metal rings 2 is inspected. The metal rings 2 which pass a quality control inspection are sequentially laminated together in a stack of about 12 layers to form a laminated band which becomes a CVT belt. The steel elements 4 are inserted are consecutively attached and the CVT belt 1 is completed.
As stated above, ultrahigh strength steel is used for the metal rings 2 of the CVT belt 1. As a type of ultrahigh steel suitable for a CVT belt 1 there is maraging steel, for example, as described in Japanese Laid-Open Patent Application No. H11-117017 (1999) titled “CARBURIZATION SURFACE HARDENING OF MARAGING STEEL”. Although maraging steel is ideal for use in the CVT belt 1 because of its characteristic dynamic fracture toughness, it is not sufficient in regard to its structural impact-fatigue strength properties. In order to compensate this fatigue strength, nitriding treatment is performed, for example, as described in Japanese Laid-Open Patent Application No. H11-200010 (1999) titled “SURFACE TREATMENT OF METALLIC MULTILAYERED BELT FOR AUTOMOBILE”.
Nitriding treatment is a process in which nitrogen permeates the front surface of the steel and forms a hardening layer (hardened membrane). Although a salt bath nitriding process, a tuffride salt bath nitriding process, a plasma nitriding process, a gas nitrocarburizing process, etc. are known, generally in mass production components such as the CVT belt 1, gas nitrocarburizing is used in terms of cost. Based on this method, a surface hardening layer of about HV 400˜700 (HV=the Vickers hardness number, an indicator of the hardness of metal) is formed on the steel surface to a depth of about 8˜15 μm (micrometers).
Furthermore, as the hardening layer is formed on the front face of the metal rings 2 and because the metal rings 2 are extremely thin (at most about 0.2 mm), a flaw (herein after denoted as an “end face defect”) may often be scarred on the side end faces in the manufacturing process of the CVT belt 1.
FIG. 12 is a diagram showing an end face defect in one of the metal rings 2. For the moment, suppose that an end face defect exists in the “a” section of FIG. 12A. FIG. 12B is a microscopic enlargement diagram (magnification: around 480 times) of this “a” section and FIG. 12C is a similar pattern diagram. As shown in FIG. 12C, a defect 2b scar by some cause is observed in the end face 2a of the metals rings 2. The defect 2b is a defective portion in the hardened layer which is clearly glossy and visible as a shiny white portion (gloss mark) as compared with the non-defective portion (hatching portion).
Conventionally, the inspection for such end face defects is performed by direct visual observation with the aid of a magnifying glass. In short, the end face 2a of the metal rings 2 is held up to the light one by one by a factory worker to artificially judge the subtle differences in gloss. However, such a manual inspection method as described above is antiquated and inefficient due to the fact that human error rate is always higher than an automated process. The accepted level of variation depends greatly on each factory worker. Thus, there is a drawback in acquiring a consistent level of inspection accuracy to further improve reproduction.
Therefore, the object of the present invention is to provide a defect inspection device for metal ring end faces which enables automated inspection of end face defects in metal rings 2 along with acquiring improved inspection efficiency, superb precision and reproducibility.